The Hearing System

Question 1

What are the three sections of the ‘ear and auditory tube’?

and

What landmarks demarkate where each section begins and ends?

Answer 1

The outer ear spans from the external auricle to the tympanic membrane

The middle ear spans from the tympanic membrane to the **oval window** - but is continuous with the nasopharynx via the auditory/eustachian tube and themastoid air cells within the mastoid bone.

The **inner ear **located medially to the **oval window **and is contained within the bony labyrinth of the petrous part of temporal bone.

Question 2

Describe the important anatomical strucutres of the outer ear

Answer 2

The **auricle **is a folded ‘ear’ structure that is responsible for collecting sound into the auditory tube. The majority of the structure is composed of cartilage; but the most inferior lobule is comprised of fat.

The external auditory meatus is the hollow auditory canal that links the external environment to the middle ear - trasmitting soundwaves. It is lined by **hairy skin and cerumen (wax) glands **that prevent the maceration of skin by water. The **lateral 1/3 is cartilagenous **but the **medial 2/3 **is bony (temporal bone).

The **vagus nerves **runs posteroinferior and the **auriculotemporal nerve **(branch of trigeminal V3) runs anterosuperior to the outer ear + across the external surface of the tympanic membrane - pain in the inner ear can be referred to other divisions of V3.

The **tympanic membrane **encloses the inner ear and is the medial most structure. The functions of the tympanic membrane are discussed further elsewhere.

Question 3

What is responsible for pain of the outer ear being referred to branches of the trigeminal V3 division?

Answer 3

The vagus nerve runs posteroinferior and the auriculotemporal nerve (branch of trigeminal V3) runs anterosuperior to the outer ear + across the external surface of the tympanic membrane - pain in the inner ear can be referred to other divisions of V3.

Question 4

Discuss the structure and function of the tympanic membrane!

Answer 4

The tympanic membrane is the most medial structure of the inner ear - enclosing the outer auditory meatus/auditory canal.

The membrane vibrates at a frequency and amplitude equal to that of soundwaves in the air hitting it; its role is to transfer and transmit energy to the more internal structures of the auditory system.

The tympanic membrane is concave externally, convex internally.

On the lateral surface, an imprint of the handle of malleus can be seen. As can a **flaccid area **superior to the handle of malleus.

If you shine a light through the tympanic membrane, it produces a reflection light in te anteroinferior quadrant of the lateral surface. This is important clinically; as abnormal pressures of the ear cavities deviate this reflection of light.

Question 5

Describe the important anatomical structures and functions of the middle ear

Answer 5

The middle ear extends from the internal surface of the **tympanic membrane **to the **petral part of the temporal bone. **

The middle ear contain two anatomical parts: **epitympanic recess **and tympanic cavity proper.

The **tympanic cavity proper **contains the ossicle bones which transfer sound energy and is continuous with the nasopharynx cavity via the **auditory tube. **Communication between these two spaces is important for equalising pressures between the middle ear and environment. Blockage of the auditory tube can prevent equalisation of pressures. However, any bacteria in the nasopharynx can communicate to the middle ear - this is a common source of infection.

The tympanic cavity proper is susceptible to inflammatory oedema that can dampen the vibrations and transference of sound energy.

The **epitympanic recess **communicates posteriorly to spaces in the mastoid bone (mastoid air cells). Inflammation of the middle ear can drain into these spaces - when this occurs it is unable to be removed resulting in subsequent pathology of these sinuses.

Question 6

Compare and contrast the differences in infant and adult auditory/eustachian tubes

Answer 6

In infants, the auditory tube is significantly more horizontal

This makes it:

• easier for microbes to spread from the nasopharynx to the middle ear
• **difficult for the the draining **of the middle ear

compared to adults.

This is contribute significantly to the high occurance of otitis media infections in infants

Question 7

How many ossicles are present in the middle ear? List them from lateral to medial.

Answer 7

Malleus, incus and stapes

All of these bones are significantly smaller than a 5c piece

Question 8

What are the actions of stapedius and **tensor tympani **in the middle ear?

Answer 8

Both of these muscles insert onto ossicles of the middle ear - they act to dampen the amplitude of sound energy transmitting through the bony ossicles.

The **Stapedius **inserts onto stapes. It is innervated by CN VII.

Tensor Tympani inserts onto malleus. It is innervated by CN V

Question 9

What nerve is observed running through the tympanic cavity proper?

Answer 9

**Chorda tympani **is a branch of the **facial nerve (CN VII) **that runs through the middle ear having origiinated from the taste buds of the anterior tongue.

Question 10

Describe the course of the facial nerve in reference to the middle ear

Answer 10

The facial nerve travels in the facial canal (via the internal accoustic meatus) that runs:

1. Superior to the middle ear; then
2. Posterior to the middle ear; before
3. Emerging through the stylomastoid foramen

Question 11

Describe the course of the internal carotid artery as it relates to the middle ear

Answer 11

The **interior carotid arteries **travel directly inferior to the middle ear.

When you have middle ear fluid retention - the throbbing that is felt is a result of the internal carotid artery pulsations being transferred to the fluid.

Answer 12

The bony labyrinth is the rigid outer wall of the inner ear. It consists of three parts: the vestibule, semicircular canals, and cochlea. These are tortuous cavities hollowed out of the substance of the bone, and lined by periosteum. They contain a clear fluid, the perilymph, in which the membranous labyrinth is situated.

The membranous labyrinth is a closed sac filled with endolymph which is bathed by the perilymph of the bony labyrinth

The **oval window **(or vestibular window) is a membrane-covered opening which leads from the middle ear to the vestibule of the inner ear. The stapes is intimately contacting the oval window.

The round window is a membrane covered opening between the middle ear and inner ear by the round window membrane, which vibrates with opposite phase to vibrations entering the inner ear through the oval window

Question 13

Describe the importance of the various segments of the membranous labyrinth

Answer 13

There are **3 semicircular ducts **orientated in discrete planes: anterior, posterior and horizontal. Containing fluid that moves with head movement and hair cells, it measures/identifies iinformation about movement.

There are three enlarged and specialised regions of the semicircular ducts that contain sensory receptors of movement:

• **Ampulla: **sensory receptors of dynamic movement (understands our head movements/directions of movement)
• **Utricle and Saccule: **sensory receptors for static equilibrium (understanding the position of our head)

The **vestibular nerve **originates from these three specialised regions

The **cochlear duct **contains sensory receptors for hearing and gives rise to the cochlear nerve

The vestibular and **cochlear nerves **join to form the vestibulocochlear nerve (CN VIII) which enters the cranial cavity at the internal accoustic meatus

Question 14

How do features of sound waves relate to the domains of sound that we percieve?

Answer 14

**Wavelength = pitch **

(higher pitch = shorter wavelength; lower pitch = longer wavelength)

Amplitude = loudness

(louder = higher amplitude; quieter = lower amplitude)

Waveform = tone/timbre

(the type of wave - e.g. sinusoidal or other)

Question 15

What is the function of the ossicles?

Answer 15

The mechanical stiffness of the ossicle chain acts to **compensate **for the difference in impedance between air and fluid environments - this is impedance matching

Impedance** **can be thought of as a medium’s resistance to movement - particles must vibrate/move for soundwaves to propagate.

Normally when sound passes from air to fluid, most is reflected - the ossicles minimise this by impedence matching.

Impendence matching is achieved by :

• 200x pressure of ossicles at the tympanic membrane compared to the inner ear
• Relative size of tympanic membrane to inner ear (20:1)
• Lever action of ossicles (1:3:1)

Question 16

Discuss the importance of the **basilar membrane **in transmitting sound waves into neural signals.

Answer 16

The basilar membrane is a stiff structural element that runs along the coil of the cochlea. It is wider at it’s apex but stiffer at its base against the oval window.

The transmission of sound energy through the oval window results in movements of the basilar membrane up and down.

The base responds to high frequencies; and the **apex responds to low frequencies. **This organisational pattern represents tonotopy - the progressive coding of sound frequencies by different cells along the membrane.

Question 17

Discuss the importance of the Organ of Conti and tectorial membrane in transmitting sound waves into neural signals

Answer 17

The **organ of Corti **is the specialised epithelium resting on the basilar membrane. It is composed of inner and outer hair cells and the tectorial membrane

The hair cells are attached to the tectorial membrane via ~ 100 stereocilia and anchored to the basilar membrane.

Sound-induced vibrations of the basilar membrane exert sheer forces on the stereocilia that causes them to bend - in turn opening ion channels that result in neuronal signaling.

Question 18

Discuss the mechanoelectrical transduction mechanism of hair cells.

Answer 18

The sheer forces exerted on hair cells by vibrations of the basilar membrane causes the deflection of hair cells towards the tallest stereocilium. This causes the opening of K⁺ channels - leading to depolarisation of the hair cell.

Depolarisation of the hair cell opens the voltage dependent Ca²⁺channels - leading to the release of glutamate that, in turn, depolarises the afferent fibre.

Displacement of the hair bundle in any direction alters the membrane potential; but this effect is asymmetrical - there are larger depolarisations than hyperpolarisations.

Question 19

What is the importance of potassium and it’s concentrations within the cochlear?

Answer 19

K⁺ acts to both depolarise and repolarise hair cells.

The cochlear chamber scala media contains endolymph fluid that is in contact with the apical surface of hair cells. The endolyph contains a high concentration of K⁺ (80mv potential).

The cochlear chamber scala tympani contains perilymph with low concentration of K⁺(0mv potential).

It is the concentration gradient of K+ between these two chambers that facilitates the movement of K⁺into the hair cells. This concentration difference is thought to be established by selective ion secretion and absorption of the **stria vascularis **

Question 20

What is the difference between inner and outer hair cells?

Answer 20

Inner hair cells

Provide auditory information for 95% of nerves that project to brainstem nuclei

Outer hair cells

Recieve efferent inputs from the superior olivary complex (brainstem) that amplify the movements of the basilar membrane to low intensity stimuli.

These hair cells change in length during low intensity stimuli - accentuating the movement of the basilar membrane - thus amplifying the signal recieved and transmitted by the inner hair cells.

• depolarisation = cell contraction
• hyperpolarisation = cell elongation

Antibiotics (aminoglycosides) and asprin can cause loss of OHC’s and loss of hearing - without OHC’s, basilar membrane movement is 100x less

Question 21

List the sequential structures in the auditory pathway that sound information traverses

Answer 21

Ear

• Hair cells (in cochlear)
• CN VIII (vestibulocochlear nerve)
• Spiral ganglion

Medulla

• Cochlear nucleus
• Superior olive
• Lateral lemniscus

Midbrain

• Inferior colliculus

Thalamus

• Medial geniculate nucleus

Temporal Lobe

• Auditory cortex

Question 22

Where does the process of determining the direction of heard sound occur?

Answer 22

The superior olivary complex is central to the determination of a sounds direction.

The superior olivary complex is comprised of:

1. medial superior olives: localisation of sound by measuring time delay (low frequencies)
2. lateral superior olives: localisation of sound by sensing intensity differences (high frequencies)
3. trapezoid body: inhibitory communication between contralateral lateral superior olives to assist in determining intensity differences

Question 23

Discuss the Duplex Theory of sound localisation (of the horizontal plane)

Answer 23

1. **The time that sound arrives at each ear **(low frequency sounds)

Determined by the length of path through the medial superior olive that the right and left ear meet. The neuron innervated by both right and left will be closer to the ear opposite to the sound origin due to delay.

2. **Interaural intensity difference **(high frequency sounds)

Is the relative excitation or inhibition to the brain that arises from circuitry of the lateral superior olives. There is contralateral inhibition of lateral superior olives by trapezoid bodies. The stronger signalling determines sound origin.

Question 24

Where is the auditory cortex?

Answer 24

The auditory gyrus (known as Herschls gyrus) is located in the temporal lobe.

The neurons within this cortex are sharply tuned for sound frequency and are organised into columns tuned for the same frequency. There are alternating regions of excitatory input from one ear and inhibitory input from the other ear on each left and right auditory gyrus.

Question 25

What type of sounds are dominantly processed asymmetrically in the auditory cortexes?

Answer 25

Speech sounds: left hemisphere

Environmental sounds: both hemisphere

Music sounds: right hemisphere

Question 26

What occurs in sudden sensorineural hearing loss?

Answer 26

Sudden sensorineural hearing loss is characterised by the sudden loss of hearing of atleast 30dp within a period of 3 hours to 3 days.

Incidence: 5-20 / 100,000

Only 10-15% known causes:

• Peripheral causes: meningitis, metastasis, acoustic neuroma, Guillian-Barre
• Central causes: MS
• Cochlear causes: Infections (esp. HSV), autoimmune disease, traumatic, metabolic, vascular, ototoxicity